In any wireless communication system, the transmitted signal is distorted due to the dynamic properties of the wireless channel. These dynamics leads to a frequency selective channel. Therefore, at the receiver side, some kind of equalization scheme can be applied in order to compensate for dynamics of the wireless channel. An ideal compensation would cancel the effects of the radio channel and make the resulting equalized channel completely frequency-flat. However, such a scheme would in most, cases lead to unwanted noise amplification which limits the performance. The equalization scheme should also suppress interference by decorrelation of the receiver antennas. Equalization schemes must therefore provide a trade-off between interference suppression, noise amplification and making the equalized channel frequency-flat.
In third generation cellular systems (including both WCDMA and CDMA2000), direct sequence code division multiple access (DS-CDMA) is adopted as multiple access scheme. CDMA is a spread spectrum technique that uses specially designed spreading sequences to spread symbol-level data to higher bandwidth chip-level sequences. One notable advantage of CDMA is its ability to exploit the multipath diversity of the wireless channel by combining the different propagation delays of the received signal. This is possible when the spreading sequences are selected in such a way that their autocorrelation function is (or at least approximately is) zero for time shifts different from zero. The most commonly used receiver for CDMA over multipath channels is the Rake receiver (2, 2A, 2B). The Rake receiver is so named because its structure resembles a garden rake, where each rake finger collects the energy corresponding to a certain propagation delay (5A-B, 5C-D, 5E-F and 5G-H).
FIG. 1A illustrates a CDMA multi-user receiver with N receive antennas that employs Rake receivers for each of the M users. The N antenna branches are coupled to an RF front-end (RX) which encompasses circuitry for transforming the received signals to baseband. Hence, the RF front-end outputs N antenna input streams, which are used as inputs to M user-specific Rake receivers (2A, 2B). FIG. 2A illustrates the conventional Rake receiver that is employed for each user in the multi-user receiver of FIG. 1A. Each of the N input streams are fed to a Rake receiver and assigned to one or multiple Rake fingers (5A-H). The Rake fingers are allocated by the path searcher (4A), which analyzes a power delay profile of the received signal and assigns a finger to each multipath component with an energy level above a certain threshold. The main function of each finger is to despread its multipath component from chip-level back to symbol-level. After despreading, the symbol-level outputs of the Rake fingers are combined by a maximal ratio combiner (MRC). The MRC (6A) is an optimum combiner which weights the symbol-level output symbols in proportion to the signal power of each finger. In the MRC, a weight estimation unit (WEU) extracts a pilot sequence from the despread signal of each finger to determine the MRC weight.
The conventional Rake receiver is optimal for demodulating a COMA signal in the presence of white noise. However, in presence of multi-user interference (MUI), normally encountered in cellular systems, the noise may be colored and the RAKE receiver is no longer optimal and may even be very far from the optimal receiver. A better solution in this case would be to employ an MMSE-optimized Rake or Generalized Rake (G-Rake) receiver for each user. An example of a MMSE-Rake/G-Rake receiver for N receive antennas is illustrated in FIG. 2B.
The MMSE-Rake and G-Rake receivers (3, 3A, 3B) have a similar structure to that of the conventional Rake receiver (2, 2A, 2B). There are, however, some details that differentiate them from the conventional Rake receiver. First, the number of Rake fingers, determined by the path searcher (4B), may be larger than the number of multipath components indicated on the power delay profile. Second, the weight estimation unit (WEU-2) needs to take ail fingers into account when the weights used for MRC (6B) are computed. Hence, the weight estimation unit (WEU-2) of an MMSE-optimized Rake or G-Rake is considerably more computationally intensive than for the conventional Rake receiver.
The MMSE-optimized Rake receiver offers improved performance over a conventional Rake receiver at the cost of a more computationally intensive implementation. Hence, in a receiver node with limited computational capabilities one may only afford to use MMSE-Rake or G-Rake receivers for a few prioritized users, while remaining users have to accept the lower level of service offered by the conventional Rake receiver.